1. Field of the Invention
The present invention relates to a housing for a seismic sensing element, and in particular to such a housing intended to be disposed on the earth's surface. The term “earth's surface” as used herein includes the sea-bed, land, and the transition zone. The present invention also relates to a seismic sensor comprising one or more seismic sensing elements disposed within a housing according the present invention.
A seismic sensor intended to be disposed, in use, on the earth's surface generally comprises one or more seismic sensing elements disposed within a housing. When the sensor is disposed on the earth's surface, the coupling of the seismic sensing element(s) to the earth is provided by the housing of the sensor; the housing also provides physical protection for the sensing element(s). Sensors of this general type are used by attaching the sensor housings to a support cable at intervals along the length of the cable. The support cable is provided with electrical leads to enable output signals from the sensors to be transmitted to monitoring and/or recording equipment, and to enable power to provided to the sensors. In the case of sea-bed seismic data acquisition, the cable is then lowered onto the sea-bed to deploy the sensors at their desired locations on the sea-bed.
Sea-bed seismic sensors generally record the pressure and the elastic wavefield of the seismic data. The pressure is a scalar quantity, whereas the elastic wavefield is a vector quantity and it is therefore necessary to measure the components of the elastic wavefield in three non-coplanar directions. The three directions chosen are usually the x-direction (defined as being parallel to the cable, and also known as the “in-line” direction), the y-direction (defined as being perpendicular to the cable, and also known as the “cross-line” direction), and the z-direction (vertical). In total, therefore, four components of the seismic data are measured. Four-component seismic data recording at the sea-bed has proven to be a very successful method for imaging through gas saturated overburdens and for characterising hydrocarbon reservoirs through lithology identification and fluid discrimination. The 3-component data for the elastic wavefield are especially useful, since they enable the separation of the compressional P-waves from the shear S-waves.
2. Description of the Related Art
Reliable interpretation of the elastic wavefield is possible only if the three components of the wavefield are recorded accurately. All of the seafloor multi-component recording systems available to the market today have problems meeting this objective. The principal problem that arises is the infidelity between the in-line (x-direction) and cross-line (y-direction) components of the elastic wavefield. By “infidelity” is meant that one component of the wavefield is recorded more accurately than another component of the wavefield. This problem is illustrated in FIG. 6(a).
FIG. 6(a) shows the x- and y-components of the elastic wavefield as measured by a conventional four-component sea-bed sensor when seismic energy is incident on the sensor housing at 45° to the cable direction. This acquisition geometry is shown in FIG. 6(b). Since the incident seismic energy makes equal angles to the x-direction and the y-direction, the measured x-component of the elastic wavefield should have an equal amplitude and phase to the measured y-component of the elastic wavefield. It is clear from FIG. 6(a), however, that the measured amplitude of the x-component is significantly larger than the measured amplitude of the y-component of the elastic wavefield for the shear-wave (S-wave) arrivals. Indeed, the seismic coupling between the earth and the seismic sensing elements achieved by some conventional sea-bed sensor housings delivers data with even greater infidelity between the measured x-component and the measured y-component. In some cases, the measured x-component and the measured y-component have a different frequency content. Some prior art sea-bed sensor housings also provide poor and inconsistent fidelity between the measured horizontal and vertical components of the measured seismic data. This infidelity in the sensor response to different components of the wavefield leads to inaccurate survey results when the data is processed.
One approach to overcoming the problem of vector infidelity is to design a deconvolution operator to attempt to correct the measured seismic data for the vector infidelity in measuring the components of seismic data. For example, co-pending UK patent application No 0000900.1 discloses a method of correcting seismic data for vector infidelity by generating a correction factor from data corresponding to one horizontal direction, and using this correction factor to correct data in another horizontal direction. As a further example, J. Gaiser has proposed, in “Compensating OBC data for variations in geophone coupling”, Proceedings of 68th Annual Meeting of the Society of Exploration Geophysicists (1998), pp 1429-1432, a deconvolution method for the correction of vector infidelity. In this method, the horizontal components of the measured data are corrected on the assumption that the vertical component (z-component) of the seismic data has been recorded accurately. These approaches each assume that the effects of vector infidelity on the acquired data can be accurately corrected during the processing of the data.
While the technique of applying a deconvolution operator can produce reasonable results if the vector infidelity is small, the distortion of the components of the wavefield during the measurement process is often too large for this technique to work. If the vector infidelity is large, a deconvolution operator can at best simply massage the data so they appear to be more consistent. The only satisfactory way to tackle the problem of sensor infidelity is to record the different components of the elastic wavefield vector with substantially the same accuracy—that is, to record the seismic data with high vector fidelity. If the individual components of the wavefield are recorded accurately, then any residual corrections applied during data processing will be smaller and so can be made more accurately.